It is required for circuits utilizing superconducting devices such as superconducting computers and biomagnetic measurement apparatus to sufficiently shield the exterior earth field and other magnetic disturbances. To this end, high-performance feromagnetic shields and superconducting shields have been used. In order to test the performance of these shields, a very weak magnetic field of nT(10 .sup.-9 T) or less should be detected. This invention will provide means for effectively measuring such a weak magnetic field.
Although a flux gate magnetometer is used to measure very weak magnetic fields down to 0.1 nT, magnetic fields less than 0.1 nT can be detected only by the SQUID magnetometer. Variations of the magnetic field can be detected by the SQUID magnetometer with the accuracy of fT(10 .sup.-15 T), but an absolute value of a static magnetic field can not be measured in its nature. Cabrera (Doctorial thesis, Stanford University, 1975) has developed a method for obtaining an absolute value of a static magnetic field in which a pick-up coil is rotated and then an output variation of the SQUID magnetometer is detected. This method is called a flip coil method.
In the flip coil method, measurement should be carried out while the pick-up coil is rotating. Due to this, it is difficult to avoid output background variation derived from the rotation of the pick-up coil and its supporting mechanism. In order to resolve this problem, output variations are recorded while the pick-up coil is rotated by 360.degree.C. Then the direction of the magnetic flux detection system itself is rotated by 180.degree. around the axis of the rotation and then output variations are recorded again while the pick-up coil is rotated from the reversed position by 360.degree.. As a result, differences between two output variations are regarded as net output variations generated by an external magnetic field. According to the method, the system is very complicated and the process is very difficult. Since magnitudes of the differences between two output variations are much smaller than those of two output variations, the S/N ratio is small.